1. Field of the Invention
The present invention relates to a call admission control method and a communication system to which the method is applied, and more particularly to a call admission control method and a communication system to which the method is applied in which, when resources shared by a plurality of communications are allocated to each communication, the priority levels in resource allocation differ between communication type.
2. Description of the Related Art
In general, when a request to establish a line connection is made to start communication or to establish a line with a higher communication speed in a public communication system where many subscribers use the common facilities during communication, the resources are allocated to the communication from free resources of the facilities. After the communication is finished, the allocated resources are released for use in allocating them to other communications. In this case, the call admission control function checks if resources satisfying the request are available and, if there are not enough resources satisfying the request, rejects the request to maintain the quality of communication that is being executed.
In some cases, whether or not free resources are available may be checked simply by checking whether or not a line is being used. In some other cases, for example, in a packet communication system where fluctuating traffic flows from a number of subscribers are multiplexed on a statistical basis, it is necessary to determine the acceptance of calls on a statistical basis based on the average traffic while allowing for an instantaneous fluctuation in traffic. For example, for use in a mobile communication system using a CDMA (Code Division Multiple Access) technology, which spreads user information between a wireless base station and a mobile station over a wireless spectrum that is broader compared to the transmission speed, Japanese Patent Laid-Open Publication No.Hei 8-191481 “Call Admission Control Method and Device” and WO98/30057 “Call Admission Control Method and Mobile Station Unit in CDMA Mobile Communication System” disclose a method for checking the current traffic road to determine if a new call will be accepted.
Japanese Patent Laid-Open Publication No.Hei 8-191481 discloses a method for determining the acceptance of a new call based on the predicted value of an interference amount made by a base station. The publication describes in detail that the interference amount on the uplink in a CDMA system is an observation amount corresponding to the traffic.
On the other hand, WO98/30057 discloses a method for determining the acceptance of a call in a mobile station by using information on the interference amount on the uplink or the amount of transmission power on the downlink reported by a base station. The publication describes in detail that, on a CDMA downlink, the total amount of transmission power of the base station is an observation amount corresponding to the traffic. In either case, the traffic may be identified in many communication systems in some way or other and a call admission control method based on the measured traffic is used.
Recently, in addition to the traditional voice communications, various types of services are provided in the world of communication including video phones, data transmission, or Internet connection. They differ not only in transmission speed but also in many aspects. For example, voice communications and video phones have a strict delay requirement and permit a delay of up to only several hundred milliseconds, while electronic mail permits longer delays. Therefore, it is required that calls from those services, each with its own nature, be accepted not in the same manner but in a flexible manner with the acceptability differing from service to service. In view of this, the method disclosed in U.S. Pat. No. 3,179,021 provides a plurality of determination thresholds for a plurality of service types to appropriately determine the acceptance of calls for each service type by comparing the current traffic measurement result with those thresholds.
The main object of the call admission control method described above is to reduce the call loss probability when the traffic load is heavy by making it difficult for a many-resource-requiring call to be accepted or to increase the whole communication quality by delaying communications with less-strict delay requirements while expediting communications with strict delay requirements. However, one of the problems with the conventional call admission control method described above is that the method is applied even when the traffic is low where calls may be accepted regardless of resource requirements or delay requirements. This results in that, though enough resources are available, the call loss probability of a specific communication type increases or its delay time becomes long.